1 Answer
1

LONGER ANSWER:
The past forms you employ, told in the condition or IF clause and would in the consequence or THEN clause, do not signify past tense; they signify unreal mode. These two clauses are both cast in the non-past tense. The Reference Time of these sentences—the time you are speaking about—is therefore non-past, present+future, “from now on”. The simple past form did properly refers the pastness of what you did to that “present” Reference Time: it happened “before now”.

A past perfect like had done, however, must be related to a past Reference Time: it distinguishes an event as having taken place “before then”, not “before now”. In your second sentence the past perfect has no past time to which it may be related; the sentence is therefore ungrammatical.

What you may say is

If I told you how I have done it, it wouldn't be a surprise.

There is however no particular reason I can think of why you should use a perfect there: you are speaking of a past event, not a present state, so the simple past did is preferred.